Cured products of thermosetting resins such as epoxy resins and phenolic resins have many excellent qualities such as dimensional stability, mechanical strength, electrical insulating properties, heat resistance, water resistance and chemical resistance. However, the cured products of those resins have small fracture toughness, and may show very brittle properties. As a result, the properties may have problems in applications of a wide range. As one of methods for solving those problems, it is attempted to compound a nanosize rubber component with reactive organic compound monomers of those resins (for example, see Patent Document 1).
In general, rubbery polymers capable of using as the rubber component are previously prepared in a particle state using, for example, a polymerization method in an aqueous medium, represented by emulsion polymerization, dispersion polymerization or suspension polymerization. The method of compounding the rubber polymer with matrix resins is a method of extracting rubbery polymer particles from the aqueous medium using an organic solvent, dispersing the rubbery polymer particles in the organic solvent, and finally mixing the rubbery polymer particles dispersed in the organic solvent with those resins (for example, see Patent Document 2).
This method involved the problem that because aqueous medium-derived water remains in thermosetting resins having rubbery polymer particles dispersed therein in an amount of about 5% to 10% by weight, curing is liable to be impaired by the influence of residual water in the starting resins when curing at the time of processing the resins, in the field of applications requiring good quality.
To solve this problem, the following methods are known as a method of reducing water concentration in the thermosetting resin solution having the rubbery polymer particles dispersed therein.
(1) Molecular sieve
(2) Pervaporation method using a gas separation membrane
(3) Distillation method
Of those methods, a dehydration method of adsorbing water molecules in porous holes of molecular sieves and the like requires a large amount of molecular sieves used to the amount of water to be dehydrated. Furthermore, in the case that polymer particles are present in a material to be dehydrated as in the present invention, its regeneration is not easy, and it is considered that the method is not suitable to industrial utilization.
Furthermore, the pervaporation method using a gas separation membrane is the technology that temperature of a solution containing a polymer located on one side of the gas separation membrane is elevated to the boiling point of water to form a vacuum on other side of the membrane, thereby removing only water molecules permeating fine pores of the membrane from the solution having been contacted with the membrane. However, in the case that the separation membrane is installed in a liquid phase part, when water is removed from a solution containing rubbery polymer particles as in the present invention, the rubbery polymer particles adhere to the membrane surface, and particularly, the fine pores are clogged. As a result, dehydration effect is liable to be decreased, and to ensure sufficient dehydration rate, area of the separation membrane is required to be very large, which is not practical. Furthermore, durability of such the separation membrane is low, and the separation membrane is not suitable for industrial use.
The distillation method is a method of directly removing water. Therefore, water can be removed even from a solution containing rubbery polymer particles as in the present invention without contacting the same with a porous material, a separation membrane and the like, and the method is considered to be most suitable. However, in the case that a general azeotropic solvent such as benzene, n-hexane, toluene or xylene is used, this poses the problem that sufficient investigations are not yet made in whether or not an industrial production process of a high quality thermosetting resin composition having rubbery polymer particles dispersed therein which is the object of the present invention can be achieved, and a high quality composition can be obtained.
Specifically, a process of efficiently reducing water in the state that rubbery polymer particles are maintained in a stably dispersed state is not yet known.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-255822    Patent Document 2: WO 2005/028546 pamphlet